The invention relates generally to high power machines and more particularly to slip ring brush systems.
In high power, variable speed machines such as described by commonly assigned Runkle et al., U.S. Pat. No. 5,742,515, rotors are excited through electrically conductive slip ring assemblies. Conventional slip ring and brush assemblies transfer power at low voltage, moderate current levels and relatively high speeds. It would be useful to have assemblies that can accommodate thousands of amperes even when rotating at low speeds (about zero (0) to about one hundred (100) rotations per minute). In conventional slip ring and brush assemblies, rotating at low speeds may lead to localized overheating, marking, and deformation.
U.S. Pat. No. 1,157,885 of the London Patent Office to Brown, Boveri and Co. Ltd. describes a technique for xe2x80x9ccausing the brushes to be moved back and forthxe2x80x9d by xe2x80x9cusing a device which serves to produce a periodic rotary motion of those parts of the machine on which the brushes are mounted.xe2x80x9d To accomplish this motion, xe2x80x9cbrush bridges are connected with the output shaft of a differential gear that changes their position, and an input shaft to this gear is equipped with means for altering the central position of the brush bridge, a second input shaft to this gear being provided with means for effecting a periodic back and forth rotation with variable amplitude.xe2x80x9d Azimuthal motion is thus achieved. The resulting assembly of this technique is difficult to control and to run at high voltages.
xe2x80x9cRotary Converters,xe2x80x9d Westinghouse Electric and Manufacturing Co., Circular No. 1028, Apr. 1903, describes an embodiment wherein axial motion is achieved between brushes and a commutator on a rotor shaft by moving the entire rotor axially in an oscillatory manner. Moving the entire rotor shaft is awkward, may be harmful, requires significant power, and can limit the amplitude of motion.
Other limitations of variable high speed machines relate to the non-uniform current distribution through the brushes that contact the slip rings as well as the rotor winding leads that couple rotor windings to slip rings. Because the brushes typically are coupled by a common, single electrically conductive lead, the current varies between brushes depending upon the proximity to the conductive lead. Rotor windings are also coupled together prior to attachment to the slip rings and similar current variations result. Although stators are sometimes connected to stationary connection rings with separate leads, as shown in Lawrence, Principles of Alternating-Current Machinery, 4th Ed., McGraw-Hill Book Co. 1953, such configurations are not used for slip rings of wound rotor machines.
Thus, there is a need for a reliable, low loss slip ring brush assembly that can efficiently accommodate a high level of current and voltage and a low rotation speed without gears. It would additionally be advantageous to have a slip ring brush assembly with uniform current sharing capabilities across the brushes and slip ring.
In accordance with one embodiment of the present invention, a brush for a slip ring is actuated with an electric or pressure actuator acting as an auxiliary driver to provide relative motion between the brush and the slip ring.
In accordance with another embodiment of the present invention, current is shared by using inductance variations or by selective coupling of rotor windings to the slip rings.